Control circuit for magnetic actuator

ABSTRACT

A control circuit for a magnetically actuated die closing apparatus, featuring force responsive die closing cycle termination circuitry as well as safety features for protecting the operator and the apparatus. The safety features include an electro-optic guard and clock circuit. The initial let down force of the ram as the dies make initial closing contact with the workpiece is limited by a let-down circuit incorporated into the control circuit.

United States Patent 1191 1111 Keller et al. 1 Jan. 1974 CONTROL CIRCUITFOR MAGNETIC 3,206,960 9/1965 Moxley 72/1 ACTUATOR 2,787,313 4/1957Chernack 72/1 3,099,175 7/1963 Lawson 72/1 Inventors: J p RlchardKeller; Bruce 3,665,742 5/1972 Felt et al 72/4 Cameron Longenecker, bothof Harrsbmg imwy wmi ekR sh r l- Herbst [73] Assigneez AMP Incorporated,Harrisburg, Attorney- Donald W. Ph1ll10n & A M P Incorporated [22]Filed: May 15, 1972 [57] ABSTRACT [21] Appl 253039 A control circuit fora magnetically actuated die closing apparatus, featuring forceresponsive die closing [52 U.S. c1. 72 1, 72/430 cycle terminationCircuitry as Well as Safety features 51 1111. c1 B2111 55/00 forprotecting the operator and the pp The [58] Field of Search 72/1, 2, 4,8, 9, f y ea ures include an electro-optic guard and 72/26, 28, 29 430clock circuit. The initial let down force of the ram as the dies makeinitial closing contact with the work- [,56]' Referen Cit d piece islimited by a let-down circuit incorporated into UNITED STATES PATENTSComm 3,584,496 6/1971 Keller 72/430 10 Claims, 5 Drawing Figures CONTROLCIRCUIT FOR MAGNETIC ACTUATOR BACKGROUND OF THE INVENTION In US. Pat.No. 3,584,496 to Joseph R. Keller, there is disclosed a system foreffecting the controlled closure of dies in a material working apparatusfeaturing a magnetically actuated ram. The system disclosed in thispatent represents a marked improvement over prior systems forcontrolling die closure in material working apparatus.

The magnetically actuated die closing system and how it relates toconventional die closing systems is fully described in the Kellerpatent. However, to aid the reader in understanding the presentinvention, the following brief disclosure of die closing systemsincluding the Keller system is given below.

Prior to the Keller system die closing force control in material workingapparatus, was generally provided by controlling the initialdisplacement between the dies. Where the apparatus is to be used informing articles or in effecting a connection between a preformedarticle and another article at a relatively slow rate, the requiredforce could be manually developed. For example, the force could bedirectly applied to the handles of a hand tool which operates as leversto drive the dies in a controlled displacement. The closing of the dieswould effect an article forming operation if the dies are worked againstsheet metal stock to cut away metal in a predefined pattern leaving adesired configuration. The metal cutting step could be followed by aforming step to form the cut pattern into an article such as anelectrical terminal. The closing of the dies could also effect aconnecting operation such as the crimping of a preformed electricalterminal to a wire. In this latter case, the terminal and wire aresituated between the dies. As the dies close, portions of the terminaldeform around the wire to thereby realize the desired connection of theterminal to the wire.

To simplify the following discussion of the invention, no furtherreference will be made to the use of the material working apparatus inthe forming of an article. Further discussion will revolve around theuse of the apparatus to effect connection between a preformed articlesuch as an electrical terminal to another article such as a wire. It isto be understood, however, that the apparatus described is of the typewhich can be used either to form an article or to effect a connectionbetween two articles. Multiple use of the apparatus may be provided byinterchanging dies and/or controlling the die closing force.

Where numerous connections are to be made within a short period of time,hand tools do not permit the required rapid cycling. Therefore, the dieclosing apparatus must use an energy source for effecting die closurewhich is other than manual. More specifically, where rapid cycling ratesare required, one die may be fixed to an anvil while the other die'wouldbe fixed to a ram. Prior to the Keller'invention, the energy source fordriving the ram took the form of an inertial system wherein energy isstored in a moving part such as a flywheel or the ram itself, or apressure system wherein energy is stored in a fluid maintained underpressure in a vessel. As more fully described in the" Keller patent,each of these die closing systems create problems which affect theefficien'cy of the apparatus and the quality of the finished product.

The Keller system solves these problems by providing a magneticallyactuated die closing system. In this system, one die is fixed to ananvil while the second die is fixed to a ram. The anvil is mounted in ahousing containing a first electromagnet. A second electromagnet ismounted on the ram and made moveable therewith. The force required fordie closing is controlled by controlling the current in the coils of theelectromagnets. More specifically, the coils in the first and secondelectromagnets are wound in an aiding sense so that when current flowstherethrough the magnets attract. The closing of the magnets is impartedto the dies. The magnetically actuated die closing system permitsprecise control over the force applied to the terminal to be crimped toa wire by providing control over the velocity and acceleration of theram as well as control over the dwell time, that is, the time duringwhich the force is exerted on the terminal.

The Keller patent describes two circuit arrangements for controlling thecurrent applied to the electromagnets. In one circuit embodiment, apulse, predefined in both duration and amplitude, is applied to themagnets. In this case, the duration and amplitude of the pulse arederived from experience with a particular workpiece.

In the second embodiment of the control circuit, the connection ofarticles, which in this case, is the crimping of a terminal to a wire,is effected with a constant force through the use of a feedback system.Upon closure of the dies, a signal is developed by a force transducer.The force transducer may be any conventional transducer which converts amechanical force into a proportional electrical signal. Examples of suchtransducers are piezoelectric, and semiconductor strain gages. Thetransducer produced signal is compared to a reference signal which isproportional to a predefined force. If the die produced force is greaterthan or less than the predefined force, the current in the magnets isvaried tobring the developed force into agreement with the predefinedforce. The force is applied to the articles being connected for apredetermined dwell time through the use of a time constant circuit.

Although the control circuit described in the Keller patent providesadequate control under some conditions, they lack the efiiciency desiredand safety features needed for a large scale production line set up, Thepresent invention provides an improved control circuit which overcomesthe deficiencies of the control circuits described in the Keller patent.

SUMMARY OF THE INVENTION featuring magnetic actuation.

It is further an object of the invention to provide a control circuitwherein no manual adjustment of apparatus elements need be made eachtime a different wire size is to be crimped to a suitable terminal.

It is still another object of the invention to provide a control circuitwhich includes safety features to protect the operator from injury aswell as to protect the material working apparatus against damage due tothe activation of the apparatus when aforeign object is between thedies.

These objects are attained in accordance with the present inventionthrough the use of a unique control circuit which provides for thedeactivation of the electromagnets in response to a predeterminedcritical force between the dies.

The control circuit also includes a let-down circuit for bringing theram into initial contact with the workpiece which in the preferredembodiment is a contact to be crimped to a wire, at a relatively lowaverage velocity, whereby the ram meets the terminal with little kineticenergy. After initial contact, the let-down circuit is deactivated tocause a rapid increase in the coil current thus developing the largeforce required to connect the preformed article to another article or toform the article by cutting, forming, etc. We have determined that in acrimping operation if the ram is allowed to approach the workpiece at ahigh velocity, the impulse developed as the ram meets the terminal maycause damage thereto as well as a deterioration of the quality of thecrimp. However, if the ram is gently brought into initial contact withthe terminal, with the crimping force subsequently applied, terminaldamage is avoided and high quality crimps are realized.

To protect the operator and the apparatus from injury and damagerespectively, the control circuit is provided with a clock circuit whichtriggers to end the die closing cycle if the critical force is notreached within a predetermined time.

An additional safety feature is provided in the form of an electro-opticguard. In operation, a die closing cycle can not be initiated or ifpreviously started will be immediately tenninated if a foreign object ofat least a predetermined size and shape is placed between the dies.Thus, if an operator should inadvertently place his hand between thedies, a die closing cycle cannot be initiated, and an initiated cycle isterminated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a prospective view of amagnetic actuator for crimping terminals with a portion of an actuatorhousing section to show the electromagnet in an open position;

FIG. 2 is a view similar to that of FIG. 1 but with the electromagnetsin a closed position with the electromagnets sectioned to show themagnet coils and bearing the structure supporting the upperelectromagnet and the ram shaft of the mechanism;

FIG. 3 is a block diagram of a prior art control circuit for use with amagnetically actuated die closing apparatus;

FIG. 4 is a block diagram of the control circuit of the presentinvention;

FIG. 5 is a schematic diagram showing in detail the control circuit ofthe present invention;

DISCUSSION OF THE PREFERRED EMBODIMENTS OF THE INVENTION FIGS. 1 and 2correspond essentially to the like numbered figures in the Kellerpatent. These figures have been modified by the inclusion of a switch 33and photoresponsive elements 53 in accordance with the teachings of thepresent invention. A bench mounted press P is shown relative toterminating an open barrel electrical terminal T to a stripped insulatedelectrical lead L. The apparatus of FIGS. 1 and 2 will generally includea number of features not shown in these drawings. Specifically, thedrawings do not show a feed mechanism for feeding terminals and/or afeed mechanism for feeding stripped leads between the dies. Thesedetails have been left off for the sake of clarity since they are notessential for a full understanding of the present invention.

The press includes a frame or housing comprised of a metal base plate 12resting on the top of a work bench or the like 10. To the metal baseplate 12 is attached an upstanding support portion 18 carrying a boxlike structure in which is mounted a magnetic actuator mechanism. Themechanism is connected to a ram 20 having a die 22 on its lower end. Ascan be seen more clearly in FIG. 2, ram 20 is connected to a ram shaft24 which extends vertically up through the support structure 18. The topof the ram shaft 24 includes a flange member 26 which serves as a stopfor a spring member 28 made to surround the upper portion of the shaft24 and bear against the top of housing 18 around the aperture thereinthrough which shaft 24 travels during operation of the press. A suitableauxilary bearing shown as 29 may be provided in the top of housing 18 tosupport shaft 24 for sliding movement relative thereto. Spring member 28is a compression spring of sufficient strength to push the ram shaft andconnected structure rapidly upward to the position shown in FIG. 1 inthe absence of a force drawing the magnets together. The lower magnet 30of the die closing apparatus is fixed to a transverse plate 55 which ispart of the housing structure 18 and is thereby fixed relative to ramshaft movement. This magnet has a central aperture through which the ramshaft 24 extends, the latter being supported therein for longitudinalsliding movement relative to the housing 18 by means of a conventionalmain bearing 36. The upper magnet 32 of the die closing apparatus isfixed to the shaft by a keyed and threaded sleeve shown as 38 in FIG. 2.The upper surface of the fixed magnet 30 carries a cushion in the formof a sheet 34 of plastic such as Mylar or the like which preventssurface engagement of the two magnets during closure in the positionshown in FIG. 2 should the unit be activated with no material betweenthe dies. Sheet 34 also maintains a maximum gap between the two magnetsto prevent the magnets from sticking together.

Each of the magnets includes a recess which extends around in an angularfashion with the body of the magnet. The recesses are shown as 40 and 43in FIG. 2. These recesses contain coils WI and WII and are terminated ina suitable manner, to be described more fully below, to a power supplyand control circuit.

The plate 12 contains a fixed die 50 secured thereto in alignment withdie 22. A force transducer 52 is provided beneath die 50. This forcetransducer is coupled in a manner to be described to the controlcircuitry of the present invention illustrated generally in FIGS. 1 and2 as box 63.

The apparatus also includes switch 33, one contact of which is mountedon magnet 32 while the other contact is mounted on magnet 30. Thisswitch is associated with the let-down circuit and acts to deactivatethe circuit after initial engagement of die 22 with the terminal orother workpiece.

Further, the metal base plate 12 includes photoresponsive element 53.These elements may take the form of conventional photoconductors. On theunderside of plate 55 there is mounted light sources (not shown) forcontrolling the electrical output of the photoresponsive elements 53.Operation of these elements in relation to the control circuit of thepresent invention will be discussed more fully below.

In operation of press P, the apparatus is initially in the positionshown in FIG. 1 with die 22 in an upward position displaced from lowerdie 50. When used for crimping, a terminal T is positioned on the lowerdie with the stripped portion of the lead L positioned in the crimpportion of the terminal. Energization of coils WI and WII results in thegeneration of a magnetic field in magnets 30 and 32 which develops aforce attracting the two magnets together. Due to the fact that magnet30 is so fixed tothe support structure 18, magnet closure draws orforces ram downwardly carrying die 22 against the terminal T and againstthe lower die 50. As this occurs, the force involved is detected orsensed by the transducer 52 which develops a signal which is used toselectively terminate the die closing cycle.

FIG. 3 illustrates a prior control circuit for use with a magneticallyactuated die closing apparatus. The details of this system are fullydisclosed in the above referenced Keller patent. In operation, a powersupply 66 provides an input signal to a control circuit 68. The outputof the control circuit is used to actuate the electromagnets denotedgenerally as 70. The force created on the terminal T is converted intoan electrical signal by the force transducer corresponding to transducer52 of FIGS. 1 and 2 and shown generally as 74 of FIG. 3.

The output of the transducer is fed to a comparator 72 which comparesthe voltage proportional to the applied force to a voltage proportionalto a reference force. When these two voltages are not in agreement, anerror signal is supplied to the control circuit 68 to increase ordecrease the signal to the electromagnets to respectively increase ordecrease the magnetic force between magnets 30 and 32 until the forcedeveloped on the terminal T and die 50 corresponds to the desired force.It should be noted, however, that the reaching of the predeterminedforce does not terminate the crimping cycles. The dwell time iscontrolled by a timing circuit. Further, the circuit of FIG. 3 does notpossess safety features necessary for protecting both the apparatus andoperator.

The unique control circuit of the present invention will now bedescribed with reference to FIGS. 4 and 5. FIG. 4 is a block diagram ofthe control circuit. We have determined that excellent quality crimpsare obtained with a wide variety of wire sizes if the crimping cycle isterminated when the force on the terminal reaches a predeterminedcritical force. Based on this discovery, the control circuit is providedwith a force responsive control circuit 114 which causes termination ofa crimping cycle when the force on the terminal reaches the criticalvalue.

Generally, operation of the die closing apparatus utilizing the controlcircuit of the present invention follows the following sequence. Magnets30 and 32 are initially in their displaced position illustrated inFIG. 1. Switch 116, which may be a foot switch, for example, is closedto initiate energization of the electromagnets. As switch 116 is closed,the start circuit generates a start signal to energize the drivingcircuit 104. The start signal is present during the entire cycle. Theenergized driving circuit 104 permits current to flow from the D.C.power supply 100 through coils WI and WII of the electromagnets 30 and32. A regulated DC power supply 102 is used to energize the circuitelements of the control circuit. The output of D.C. power supply 100 issubstantially greater than the output of the regulated supply 102.Typically, power supply 100 supplies approximately 130 volts DC whilethe regulated power supply 102 supplies 30 volts D.C. The relativelyhigh potential provided by the power supply 100 is needed to produce thelarge crimping force, in a given time often in the area of 2,000 pounds,required to effect a quality crimp.

As current from the power supply 100 energizes the coils of theelectromagnets, shown generally as 70 in FIG. 4 and corresponding to theindividually shown electromagnets 30 and 32 of FIGS. l and 2, themagnets are brought together. The initial velocity at which the magnetsapproach each other is controlled by the let-down circuit 112. Inoperation, this circuit limits the average drive current to theelectromagnets to thereby bring die 22 into initial engagement with theterminal T with little kinetic energy. This protects the terminal T fromthe damage which often results if the die 22 is rapidly brought intoinitial engagement with the terminal. After initial contact is made, thelet-down circuit is deactivated thereby increasing the drive currentwhich causes the attractive force between the magnets to increase. Theincreased drive current permits the required crimping force to berapidly developed between the dies. As will be described in detailbelow, the letdown circuit operates to intermittently deactivate thedisable circuit 109 thereby intermittently interrupting the drivecurrent passing through the driving circuit I04. Intermittentinterruption of the drive current reduces its average value thuslimiting the closing force.

The force between the dies and thus the force on the terminal T isdetected by the force transducer 52 which is applied to the forcecontrol circuit N4 and transformed into a proportional potential. Theforce control circuit 114 also includes a reference potentialproportional to a critical, force, the attainment of which signitiesthat a crimp has been effected. Circuit 1'14 also includes a comparatorwhich compares the reference potential with the force transducersupplied potential and when the two correspond, generates an outputsignal which operates on the start circuit 106 to cut off the signalthus terminating the crimping cycle.

If a foreign object of at least a predetermined size and shape isaccidently positioned in the crimping area, the electro-optic guard 12]comes into action to produce an output signal which operates on startcircuit 108 to cut off the start signal.

It is possible for a foreign object such as a wrench or screwdriver toappear between the dies and not activate the electro-optic guard. Aswill be described more fully below, the control circuit includes anothersafety feature which operates to terminate the closing cycle should sucha foreign object appear between the dies.

' When the dies close on a screwdriver or wrench, for example, magnets30 and 32 are kept apart at a distance sufficient to keep the contactsof switch33 from closing. Thus, the critical force is not reached andonly a very small force is exerted on the object between the dies. Theadded safety feature for terminating a cycle is the clock circuit 110.This circuit is a timing circuit activated upon the initiation of a dieclosing cycle. If the critical force is not reached within apredetermined time, the clock circuit generates an output signal whichsignals the start circuit 106 to cut off the start signal. It alsosignals the disable circuit 109 to function. The output fromthe disablecircuit 109 causes the generation of a warning signal to notify theoperator that there is trouble in the apparatus and also preventsfurther cycles from being initiated until the control circuit ismanually reset by the operator.

Operation of the control circuit of the present invention may be morefully understood by referring to FIG. 5. Like numbered elements in FIGS.1-5 correspond to common elements. To initiate a die closing cycle whichin the specific embodiment disclosed results in a crimping cycle, switch116 is closed. Prior to the closing of switch 116, capacitor C hascharged through the 30 volt line 113 from the regulated DC power supplyline 102. The closing of switch 116 causes the potential acrosscapacitor C to appear at the anode of unijunction transistor 0,. Thistransistor may be a 2N 6027 unijunction transistor. The unijunctiontransistor becomes conductive when the anode potential is inpredetermined relationship with the gate potential. To turn thetransistor off a negative pulse may be supplied to the anode to bringthe anode potential to approximately ground potential. The applicationof the capacitor C voltage to the anode of transistor Q causes it toconduct. As a result, the potential at node a in the driving circuit 104rises thereby turning on transistor Q,,. When transistor Q,, conducts,,transistors Q, Q also conduct, to energize WI and W1] thereby creatingan attractive force between the magnets 30 and 32.

Forming part of the let-down circuit 112 is a feedback coil L,physically placed in the vicinity of coils WI and W1] whereby current inthese coils cause an induced current in the feedback coil. Switch 33 isinitially open. One terminal of feedback coil L, is connected to the 30volt line 113 through a suitable resistance while the second terminal iscoupled to the inverting input of an operational amplifier OP, connectedas a trigger circuit. Operational amplifier OP, can be a conventionalSchmitt trigger. Prior to encrgization of coil L the output potential ofthe trigger circuit OP, is low thereby retaining transistor Q, in itsnon-conducting state. As current is induced in feedback coil L, thepotential across the capacitor C previously charged from line 113,drops. When the trigger voltage is reached, the output of circuit OP,rises to a potential sufficient to turn on transistor O,,,. The emitterof 0, is tied to the base of transistor 0 in the disable circuit 109.Thus, when transistor Q, conducts a positive bias is applied to the baseof 0,, thereby turning this transistor on. As transistor 0,, turns on,the potential at the node a rapidly drops thereby turning off transistorQ Which in turn turns off transistors O Q of the driving circuit. Thisblocks further current from flowing through the coils WI and Wll. As thecurrent in the feedback coil now decreases, the potential acrosscapacitor C increases returning the output of the trigger circuit OP, toits low state causing transistors Q10 and O to turn off. As a result,the potential at node a of the driving circuit 104 again increases toturn on the driving circuit to cause current flow in coils W1 and Wll.The driving circuit 104, thus, rapidly energizes and de-energizeslimiting the average current flow in coils WI and Wll. This smallaverage current causes a small attractive force to be developed betweenthe magnets thus limiting the velocity at which the magnets 30 and 32initially approach each other. As a result, die 22 is brought gentlydown upon the terminal T. As die 22 meets the terminal, switch 33 isclosed to keep the output of the trigger circuit OP, at a low levelthereby retaining transistors Q, and 0, off.

Operation of switch 33 can best be understood by referring to FIGS. 1and 2. One contact of the switch is mounted on magnet while the othercontact on magnet 32. The contacts close when these magnets areseparated by a predefined distance. This distance corresponds to thedistance between die 22 and when die 22 makes initial contact withterminal T.

With Q off, the potential at node a rises and remains at this high levelfor a time sufficient to allow the current in coils WI and W1] to buildup to a value sufficient to cause a large attractive force to bedeveloped between the magnets. This force is of course transferred tothe ram 20 which causes the ram to come down hard on terminal T. Thus,dies 22 and 50 come together with sufficient force to create a qualitycrimp.

The force on the terminal is monitored by the force transducer 52. Theoutput of transducer 52 is coupled to the force control circuit 114. Theforce control circuit also includes an operational amplifier OPconnected as a comparator circuit. The output of circuit OP is connectedto the gate terminal of unijunction transistor Q The anode of transistor0,, is connected to a reference voltage source in the form of apotentiometer 122. The potential at the wiper of the potentiometer 122is proportional to the desired critical force. As the force on thestrain gage 52 increases, the potential thereacross increases therebyincreasing the input to the inverting side of 0P This causes a decreasein the potential at its output. When the force on terminal T correspondsto the critical force, transistor 0,, is triggered on. The cathode oftransistor 0,, is coupled to the base of transistor 0, of the startcircuit 108. Conduction of transistor Q,, biases transistor 0, intoconduction causing the anode of transistor 0,, to ap proach groundpotential turning it off. When transistor Q turns off, the potential atanode a of the driving circuit 104 drops to a value sufficient to turnoff transistor 0,, thereby disabling the driving circuit 104. In thismanner, as soon as the critical force is reached, driving current isblocked from the coils and the die closing cycle is terminated. Themagnets return to their initial open position under the control of biasspring 28. The critical force for terminating the closing cycle can beeasily varied by varying the potential with the potentiometer 122.

Let it now be assumed that a foreign object, such as a screwdriver restson die 50. The control circuit is provided with a clock circuitoperating in conjunction with the let-down circuit 112. When the startcircuit 108 is activated to cause transistor O to conduct, transistor Qof the clock circuit turns on thereby allowing capacitor C, to charge.Capacitor C, forms a portion of a time constant circuit which includesresistors 124 and 126 as well as transistor 0;. Capacitor C, continuesto charge to a threshold voltage which turns on transistor 0,. It shouldbe remembered that as capacitor C, charges, die 22 is being slowlybrought down into engagement with the screwdriver through the operationof a let-down circuit 112. The applied force is so low that no damage iscaused to the dies. Switch 33 is so arranged that its contacts will notclose when a relatively large object such as the screwdriver is restingon the die 50. This assumes that the height of the foreign object isgreater than that of the contact to be erimped. Since this is generallythe case, when die 22 makes its initial contact with the foreign object,the magnets 30 and 32 have not closed a sufficient distance to permitthe contacts of switch 33 to engage. Thus, the let-down circuit remainsoperative thereby limiting the force applied by the magnet and thecritical force will never be reached.

When the threshold voltage of transistor Q is reached, it conductsthereby causing transistor Q of the start circuit to turn on whichresults in the turning off of transistor Q The turning on of transistor0., also causes transistor O to conduct which biases transistors Q and Qof the disable circuit 109 into conduction. When Q conducts, light 111glows indicating to the operator that there is trouble in the apparatus.When Q, is on, node a is prevented from rising to the potentialnecessary to turn on the driving circuit 104 even if another die closingcycle is initiated. Transistor Q and Q remain conducting until thecontrol circuit is manually reset by the operator by closing resetswitch 115. It should be noted that even though the turning on oftransistor Q, causes transistors Q Q, to turn off, transistorQ remainsconducting. So long as Q, conducts, transistors Q and 0 remain on. Ifthe operator attempts to initiate another cycle without resetting thecircuit, no cycle will start since the cathode of Q is shorted to groundthrough conducting transistor 0-,. When switch 115 is closed, 0, turnsoff thus turning off transistor 0, and 0 allowing node a to rise to thedrive circuit turn on potential upon a subsequent closing of. switch116.

A further safety device in the form of an electro-optic guard 121 isalso included with the control circuit of the invention. This safetydevice is directed primarily to protecting the operator should heinadvertently place his hand between the dies and close switch 116.Referring to FIGS. 1 and 2, a plurality of photoresponsive elements,such as photoconductors, are mounted on the base 12 of the support 18. Alight source for energizing these photoresponsive elements may bemounted on the underside of member 55 of support frame 18. Theresistance of a photoconductor varies inversel'y with the intensity ofreceived light. Thus, when the light paths between a light source andthe photoconductors are not interrupted, the resistance of each of thephotoconductors is at its lowest value.

When the light to a photoconductor is interrupted, its

resistance increases. The photoconductors are coupled to an operationalamplifier 0P forming a portion of the electro-optic guard circuit 121.Whenever the photoconductors 53, noted individually by-PC, PC do nothave their'impinging light interrupted, their resistances are at theirlowest value and thus the output of OP 3 is at its lowest level. As thelight to the photoconductors become interrupted as would be the case ifthe operator places his hand in the area of the dies, the resistances ofsome of the photoconductors lPC PC, increase thereby increasing theoutput of 0P When a sufficient number of photoconductors possess a highresistance the potential at the anode and gate of transistor 0,, assumesa predetermined relationship necessary for triggering 0,, intoconduction. When Q turns on, transistor 0 turns on thereby eitherpreventing the driving circuit 104 from becoming activated or ifpreviously activated, causing it to become deactivated. in this manner,should the operator place his hand in the vicinity of the die, he willinterrupt the light to a suffi- 6 While the invention has beenparticularly shown and desscribed with reference to a preferredembodiment thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention.

We claim:

1. In a cyclically operated, magnetically actuated material workingapparatus comprising a stationary housing, a first electromagnet affixedto said housing, a ram movable with respect to said housing, a secondelectromagnet afi'rxed to said ram and movable with said ram, said ramand said second electromagnet having a first position when saidelectromagnets are deenergized, said first and second electromagnets andsaid ram being positioned so that said first and second electromagnetsare magnetically attracted towards each other when energized to impart aforce to said ram to cause said ram to move along a predetermined pathduring each cycle of operation of said apparatus, a first die positionedat a first end of said ram, and a second die secured to said housing ata distance from said first die when said electromagnets are deenergizedand in a position to close with said first die when said electromagnetsare energized; a control means for controlling the characteristics ofmotion and force of said ram along said predetermined path when saidelectromagnets are energized;

said control means constructed to respond to the position of said ramalong a first portion of said predetermined path to maintain the forceand velocity of movement of said ram at a predetermined low rate;

said control means further constructed to respond to the position ofsaid ram along a second portion of said predetermined path to create aprogressively increasing attractive force between said first and secondelectromagnets as said ram moves said first die into close proximity tosaid second die;

said control means further constructed to respond immediately to theattainment of a predetermined force upon the material, which is beingworked and which is positioned between said first and secondelectromagnets, after contact of said first and second dies with saidmaterial to deenergize said electromagnets; and

means for returning said ram to said first position when saidelectromagnets are deenergized.

2. In the apparatus of claim 1 in which said housing comprises a worksurface upon which is secured said second die' and in which said controlmeans further comprises:

a plurality of light sensitive devices mounted upon said housingand alight source also mounted upon said housing;

said light sensitive devices and said light source being positioned toenable the interception of light between said light source and saidlight responsive devices by an object, such as a person's hand, placedupon said work surface to abort said cycle of operation and deenergizesaid electromagnetic means.

3. A control circuit in accordance with claim 1, comprising:

means for initiating a cycle of operation of said apparatus; and

timing means responsive to the initiation of a cycle of operation ofsaid apparatus to abort said cycle of operation if said cycle ofoperation is not completed in a predetermined interval of time.

4. In the apparatus of claim 1 in which said housing comprises a worksurface upon which is secured said second die and in which said controlmeans is constructed to cause said first portion of said predeterminedpath to be of a length to cause said first die to come into sufficientlyclose proximity to said work surface to make contact with small tools,such as screw drivers, which are present upon said work area surface,and to have the progress of said ram halted thereby.

5. A magnetically actuated material working apparatus comprising:

a housing,

a ram means mounted in a given position on said housing and movablethrough a predetermined P a first die mounted on said housing;

a second die mounted on said ram means and positioned to close upon saidfirst die when said ram means is moved through said predetermined path;

electromagnetic means constructed when energized to move said ram meansalong said predetermined path during each cycle of operation;

a control circuit means for controlling the characteristics of speed andforce of said ram means as it moves along said predetermined path;

said control circuit means comprising a first control sectionconstructed to restrain the average velocity and force of said ram meansover a first portion of said predetermined path below predeterminedlevels;

said control circuit means further comprising a second control sectionconstructed to respond to the passing of said ram means past a certainpoint along said predetermined path to progressively increase the forceimparted to said ram means by said electromagnetic means above saidpredetermined level to a degree sufficient to work said material; and

said control means further comprising a third control sectionconstructed to respond to the attainment by said ram means of apredetermined maximum force in the direction of said predetermined pathto deenergize said electromagnetic means; and

means for returning said ram means to said given position to complete acycle of operation.

6. The apparatus of claim 5 in which said housing comprises a worksurface upon which is secured said first die and in which said controlcircuit means further comprises:

a plurality of light sensitive devices mounted upon said housing and alight source also mounted upon said housing;

said light sensitive devices and said light source being positioned toenable the interception of light between said light source and saidlight responsive de- 8. The apparatus of claim 5 in which said housingcomprises a work surface upon which is secured said first die and inwhich said control circuit means is constructed to cause said firstportion of said predetermined path to be of a length to cause saidsecond die to come into sufficiently close proximity to said worksurface to make contact with small tools, such as screw drivers, whichare present upon said work area surface, and to have the progress ofsaid ram means halted thereby.

9. In a cyclically operated, magnetically actuated material workingapparatus comprising a stationary housing, a first electromagnet affixedto said housing, a ram movable with respect to said housing, a secondelectromagnet afiixed to said ram and movable with said ram, said ramand said second electromagnet having a first position when saidelectromagnets are deenergized, said first and second electromagnets andsaid ram being positioned so that said first and second electromagnetsare magnetically attracted towards each other when energized to impart aforce to said ram to cause said ram to move along a predetermined pathduring each cycle of operation of said apparatus, a first die positionedat a first end of said ram, and a second die secured to said housing ata distance from said first die when said electromagnets are deenergizedand in a position to close upon said first die when said electromagnetsare energized; a method for controlling the characteristics of motion ofsaid ram along said predetermined path to obtain optimum working of saidmaterial and comprising the steps of:

moving said ram along a first portion of said predetermined path at alow rate of speed and with a low force imparted thereto; moving said ramalong a second portion of said predetermined path when said first andsecond dies are in close proximity to each other to create aprogressively increasing attractive force between said first and secondelectromagnets as said ram moves said first die closer towards saidsecond die;

deenergizing said electromagnets immediately upon the attainment of apredetermined force between said first and second electromagnets aftercontact of said first and second dies with the material being worked on;and

returning said ram to said first position when said electromagnets aredeenergized.

10. In a magnetically actuated material working ap paratus comprising ahousing, a ram means mounted in a given position on said housing andmovable through a predetermined path, first and second dies mountedrespectively on said housing and said ram means and positioned to closeupon each other when said ram is moved through said predetermined path,and electromagnetic means constructed, when energized, to move said rammeans along said predetermined path; a method for controlling thecharacteristics of speed and force of said ram means as it moves alongsaid predetermined path by controlling energization of saidelectromagnetic means to obtain optimum working of said material andoptimum safety for the human operator, and comprising the steps of:

restraining the average velocity and force of said ram means to valuesbelow predetermined levels as said ram means moves over a first portionof said predetermined path;

between said dies and said material therebetween as said dies movetowards each other while forming the said material positionedtherebetween;

deenergizing said electromagnetic means immediately in response to thedetection of said predetermined force; and

returning said ram to said given position.

1. In a cyclically operated, magnetically actuated material workingapparatus comprising a stationary housing, a first electromagnet affixedto said housing, a ram movable with respect to said housing, a secondelectromagnet affixed to said ram and movable with said ram, said ramand said second electromagnet having a first position when saidelectromagnets are deenergized, said first and second electromagnets andsaid ram being positioned so that said first and second electromagnetsare magnetically attracted towards each other when energized to impart aforce to said ram to cause said ram to move along a predetermined pathduring each cycle of operation of said apparatus, a first die positionedat a first end of said ram, and a second die secured to said housing ata distance from said first die when said electromagnets are deenergizedand in a position to close with said first die when said electromagnetsare energized; a control means for controlling the characteristics ofmotion and force of said ram along said predetermined path when saidelectromagnets are energized; said control means constructed to respondto the position of said ram along a first portion of said predeterminedpath to maintain the force and velocity of movement of said ram at apredetermined low rate; said control means further constructed torespond to the position of said ram along a second portion of saidpredetermined path to create a progressively increasing attractive forcebetween said first and second electromagnets as said ram moves saidfirst die into close proximity to said second die; said control meansfurther constructed to respond immediately to the attainment of apredetermined force upon the material, which is being worked and whichis positioned between said first and second electromagnets, aftercontact of said first and second dies with said material to deenergizesaid electromagnets; and means for returning said ram to said firstposition when said electromagnets are deenergized.
 2. In the apparatusof claim 1 in which said housing comprises a work surface upon which issecured said second die and in which said control means furthercomprises: a pluraliTy of light sensitive devices mounted upon saidhousing and a light source also mounted upon said housing; said lightsensitive devices and said light source being positioned to enable theinterception of light between said light source and said lightresponsive devices by an object, such as a person''s hand, placed uponsaid work surface to abort said cycle of operation and deenergize saidelectromagnetic means.
 3. A control circuit in accordance with claim 1,comprising: means for initiating a cycle of operation of said apparatus;and timing means responsive to the initiation of a cycle of operation ofsaid apparatus to abort said cycle of operation if said cycle ofoperation is not completed in a predetermined interval of time.
 4. Inthe apparatus of claim 1 in which said housing comprises a work surfaceupon which is secured said second die and in which said control means isconstructed to cause said first portion of said predetermined path to beof a length to cause said first die to come into sufficiently closeproximity to said work surface to make contact with small tools, such asscrew drivers, which are present upon said work area surface, and tohave the progress of said ram halted thereby.
 5. A magnetically actuatedmaterial working apparatus comprising: a housing, a ram means mounted ina given position on said housing and movable through a predeterminedpath; a first die mounted on said housing; a second die mounted on saidram means and positioned to close upon said first die when said rammeans is moved through said predetermined path; electromagnetic meansconstructed when energized to move said ram means along saidpredetermined path during each cycle of operation; a control circuitmeans for controlling the characteristics of speed and force of said rammeans as it moves along said predetermined path; said control circuitmeans comprising a first control section constructed to restrain theaverage velocity and force of said ram means over a first portion ofsaid predetermined path below predetermined levels; said control circuitmeans further comprising a second control section constructed to respondto the passing of said ram means past a certain point along saidpredetermined path to progressively increase the force imparted to saidram means by said electromagnetic means above said predetermined levelto a degree sufficient to work said material; and said control meansfurther comprising a third control section constructed to respond to theattainment by said ram means of a predetermined maximum force in thedirection of said predetermined path to deenergize said electromagneticmeans; and means for returning said ram means to said given position tocomplete a cycle of operation.
 6. The apparatus of claim 5 in which saidhousing comprises a work surface upon which is secured said first dieand in which said control circuit means further comprises: a pluralityof light sensitive devices mounted upon said housing and a light sourcealso mounted upon said housing; said light sensitive devices and saidlight source being positioned to enable the interception of lightbetween said light source and said light responsive devices by an objectsuch as a person''s hand placed upon said work surface to abort saidcycle of operation and deenergize said electromagnetic means.
 7. Theapparatus of claim 5 in which said control circuit means furthercomprises: means for initiating a cycle of operation of said apparatus;and timing means responsive to the initiation of a cycle of operation ofsaid apparatus to abort said cycle of operation if said cycle ofoperation is not completed in a predetermined interval of time.
 8. Theapparatus of claim 5 in which said housing comprises a work surface uponwhich is secured said first die and in which said control circuit meansis constructed to cause said first portion of said predetermined path tobe of a length to cause saiD second die to come into sufficiently closeproximity to said work surface to make contact with small tools, such asscrew drivers, which are present upon said work area surface, and tohave the progress of said ram means halted thereby.
 9. In a cyclicallyoperated, magnetically actuated material working apparatus comprising astationary housing, a first electromagnet affixed to said housing, a rammovable with respect to said housing, a second electromagnet affixed tosaid ram and movable with said ram, said ram and said secondelectromagnet having a first position when said electromagnets aredeenergized, said first and second electromagnets and said ram beingpositioned so that said first and second electromagnets are magneticallyattracted towards each other when energized to impart a force to saidram to cause said ram to move along a predetermined path during eachcycle of operation of said apparatus, a first die positioned at a firstend of said ram, and a second die secured to said housing at a distancefrom said first die when said electromagnets are deenergized and in aposition to close upon said first die when said electromagnets areenergized; a method for controlling the characteristics of motion ofsaid ram along said predetermined path to obtain optimum working of saidmaterial and comprising the steps of: moving said ram along a firstportion of said predetermined path at a low rate of speed and with a lowforce imparted thereto; moving said ram along a second portion of saidpredetermined path when said first and second dies are in closeproximity to each other to create a progressively increasing attractiveforce between said first and second electromagnets as said ram movessaid first die closer towards said second die; deenergizing saidelectromagnets immediately upon the attainment of a predetermined forcebetween said first and second electromagnets after contact of said firstand second dies with the material being worked on; and returning saidram to said first position when said electromagnets are deenergized. 10.In a magnetically actuated material working apparatus comprising ahousing, a ram means mounted in a given position on said housing andmovable through a predetermined path, first and second dies mountedrespectively on said housing and said ram means and positioned to closeupon each other when said ram is moved through said predetermined path,and electromagnetic means constructed, when energized, to move said rammeans along said predetermined path; a method for controlling thecharacteristics of speed and force of said ram means as it moves alongsaid predetermined path by controlling energization of saidelectromagnetic means to obtain optimum working of said material andoptimum safety for the human operator, and comprising the steps of:restraining the average velocity and force of said ram means to valuesbelow predetermined levels as said ram means moves over a first portionof said predetermined path; detecting when said second die comes intothe immediate proximity of said material to be worked; progressivelyincreasing the force imparted to said ram means by said electromagneticmeans to a degree sufficient to work said material in response to thedetection of said immediate proximity of said second die to saidmaterial; detecting the attainment of a predetermined force between saiddies and said material therebetween as said dies move towards each otherwhile forming the said material positioned therebetween; deenergizingsaid electromagnetic means immediately in response to the detection ofsaid predetermined force; and returning said ram to said given position.